This invention relates to cellular telephony and, more particularly, to the use of packet techniques in cellular telephony.
The number of people that use cellular telephones is continually increasing. Because the available bandwidth is controlled by governmental regulations, providers of cellular telephony are meeting the increase in users by establishing smaller cell sizes. Smaller cell sizes accommodate larger numbers of mobile units within the same overall bandwidth because smaller cell sizes effectively increase the rate of bandwidth re-use per unit area. However, as cell sizes shrink, mobile units move between cells more frequently. In a circuit switched system, each move requires that one circuit be torn down and another one set up. Consequently, as cell sizes decrease, the work associated with handing off users between cells increases. In addition, when a mobile unit traverses more cells during its connection, it is more likely that the mobile unit will encounter a cell with more units than the bandwidth can support.
Packet switching, as compared to circuit switching, reduces the work required for hand off because addresses embedded within the packets are used to route individual packets rather than setting up and tearing down circuits. Packet switching was used in early military cellular systems. Those networks were designed to be rapidly deployed, were aimed primarily for wireless interconnection between mobile units, and were not connected to a wired backbone network.
Currently, the prevalent commercial cellular system in the United States is a circuit switched arrangement that employs Time Division Multiplexing (TDM). Another system, which is also a circuit switched system, employs Code Division Multiple Access (CDMA). These cellular systems can transmit data in the form of packets, but that does not constitute xe2x80x9cpacket switching,xe2x80x9d either in the sense employed in the aforementioned military system or in the sense employed in this disclosure. Specifically, while the data may have a packet format, the switching within the cellular environment is not based on the explicit address information in the packets. For example, in TDM the address is implicit in the frequency and time slot at which the mobile unit operates. The explicit addressing characteristic of packet switching is more flexible than implicit addressing. With explicit addressing, the capacity on the shared medium can be reassigned as required and the destination can be changed without advance notice. Because of that, it is beneficial to fashion a packet switching approach for cellular communication that interfaces effectively with a wired backbone network.
An improved system is achieved in a cellular arrangement where mobile units employ a moveable slot TDM approach to send packets to base stations, and where the base stations use a non-contention approach. More specifically, from among a specified band of frequencies, the base station selects a frequency (an outbound frequency) and uses it to transmit information packets to a mobile unit within the cell covered by the base station. The information packets are simply queued with information packets destined to other mobile units, as necessary, and transmitted over the outbound frequency. Corresponding to the outbound frequency there is an inbound frequency that is used by the mobile unit to transmit information packets to the base station.
The base station also transmits information about the status of the inbound frequency; i.e., information that informs whether the inbound frequency is occupied by a signal that is being transmitted to the base station, or whether a collision exists. A collision on an inbound frequency occurs when more than one signal is simultaneously transmitted on the inbound frequency. This information can be communicated over a separate channel. It can also be embedded within the stream of bits on the outbound frequency by appropriate injection of Data Link Escape (DLE) sequences.
In addition to transmitting information packets and embedded DLE sequences over the outbound frequencies, the base station transmits general xe2x80x9ccontrol-channelxe2x80x9d information, in a TDM fashion, over a frequency that is shared by all base stations.
The mobile units, on the other hand, employ a movable slot TDM (MSTDM) protocol in their transmissions to a base station. Before it transmits, each mobile unit listens to a signal from the base station that informs it of the status of the in-bound frequency that the mobile unit is assigned to use for transmissions to the base station. If the DLE sequences inform a mobile unit that the inbound frequency is not busy (when such an approach is used), the mobile unit is permitted to begin transmissions. When the mobile unit wishes to transmit a data packet or a first voice packet, the mobile unit is also sensitive to collision information (e.g., delivered via the DLE sequences). When a collision is detected, such a mobile unit stops transmitting and tries again later. When the mobile unit wishes to transmit continuation voice packets, it only listens for a non-busy inbound frequency before it transmits. It does not stop transmitting in case of a collision. Continuation packets include a short header that carries no information, to insure against corruption of information in case of a collision.
With the use of MSTDM for inbound traffic, the re-use of frequencies is simplified in the system. Specifically, the only requirement is that adjacent cells should not use the same frequencies. Accordingly, the available band of frequencies (minus the xe2x80x9ccontrol channelxe2x80x9d frequency) is divided into three sub-bands, and different sub-bands are assigned adjacent cells. The control channel is broken up into time segments, and the time segments are assigned to adjacent base stations in the same way that the sub-bands are assigned.